The invention concerns a relatively small auxiliary filter. Small filters with a hydrophobic filter membrane are used to aerate and vent closed containers as well as flexible tube and pipe systems which are partially filled with liquid. Such gas filters should make possible a gas exchange between such a container and the ambient surrrounding atmosphere under sterile conditions. The gas volumes to be exchanged are relatively small, so that the filters themselves are small and the circular flat filter blanks have diameters of between about 12 to 100 mm. Such filter units are normally constructed from two plastic facing disc-like housing parts, each of which has an inlet and an outlet and a channeled filter support. The channels provide communicating pathways to the inlet and the outlet. A hydrophobic filter membrane in the form of a flat disc filter is clamped in between annular flanges of the two housing parts so that it is leak-proof about its periphery. The two housing parts are adhered or welded on their edges to a unit comparable to the housing part.
The gas filters are either integral into another filter housing in accordance with U.S. Pat. No. 3,448,011, DE-OS No. 1,549,835 and DE-GM No. 8,108,010.7 which filters liquids, or the gas filter is constructed as a separate, attachable unit in acordance with DE-OS No. 3,202,330 which can be connected by means of normal connecting adapters to hose systems or filter units.
A great variety of filter supports have been developed for such small filters, whether they were for filtering liquids or gases, in order to achieve optimum filtration performance. The filter supports which were developed for pure liquid filtration and are equipped with a hydrophilic filter membrane are generally also suitable for the filtration of gases if the filter support is equipped with a hydrophobic filter membrane. Problems only occur if the liquid level of a partially filled liquid reservoir to be aerated or vented momentarily rises to the level of the filter support itself or directly to the level of the hydrophobic membrane and fills up some of the channels of the filter support. This can happen, for example, if the liquid in filter housings in accordance with the state of the art revealed in DE-GM No. 81 08 010.7 and DE-OS No. 1,549,835 gets into the filter support or if the liquid in a closed flexible tube systems filled with medicinal solutions, e.g. infusion solution or blood, rises due to changing pressure conditions in the conduit system to the filter level of the hydrophobic filter. There is a danger in this instance that the hydrophobic membrane becomes blocked and that this brief blockage becomes permanent, since as a consequence of the filter construction the trapped liquid usually can not leave the free spaces of the filter support on account of capillary forces caused by the surface tension of the liquid.